WASHINGTON (AP) -- US senators exasperated with the Obama administration's response to Russia's aggression in Ukraine are threatening to act unilaterally on new sanctions.

Lawmakers challenged senior administration officials at a hearing Wednesday, describing the current penalties against Moscow as feckless and demanding the U.S. act swiftly on another round of sanctions.

Sen. Bob Menendez, the chairman of the Foreign Relations Committee, said he did not know how long Congress would wait before moving independently.

The Democrat said the administration promised new sanctions a month ago but nothing happened. More than 400 people have died and thousands have fled their homes over nearly three months in Ukraine as fighting continues between government forces and pro-Russia separatists.